1. Field of the Invention
This invention relates to a radiation image read-out apparatus. This invention particularly relates to a radiation image read-out apparatus, wherein a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is detected and photoelectrically converted by a photodetector.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (i.e.-, an image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Further, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In order for an image signal to be detected accurately, certain factors which affect the image signal should be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet, and the like. Novel radiation image recording and reproducing systems, which accurately detect an image signal, have been proposed. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order to approximately ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used for the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal, which have been adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
In a radiation image read-out apparatus, which is provided with a photodetector for photoelectrically converting the light emitted by the stimulable phosphor sheet, the amplification factor for the analog image signal obtained from photoelectric conversion in the photodetector is controlled such that an image having an appropriate image density can be reproduced. However, the speed, with which the image signal is detected from the stimulable phosphor sheet, has been increased recently. Therefore, the amount of emitted light, which enters the photodetector per unit time, has become large, and the dynamic range of the light emitted by the stimulable phosphor sheet has become wide. Accordingly, in cases where the dose of radiation delivered to the stimulable phosphor sheet is large, it often occurs that an amount of light exceeding the capacity of the photodetector is emitted by the stimulable phosphor sheet. In such cases, the photodetector becomes saturated and cannot generate an image signal corresponding to the dose of radiation delivered to the stimulable phosphor sheet.
It is considered to widen the dynamic range of the photodetector and to improve the performance of the photodetector such that the photodetector can detect a large amount of light emitted by the stimulable phosphor sheet. However, if the dynamic range of the photodetector is widened, it will become difficult to detect a small change in contrast of a reproduced image.
Also, it is considered to adjust the amount of the stimulating rays irradiated to the stimulable phosphor sheet. However, if the amount of the stimulating rays irradiated to the stimulable phosphor sheet is adjusted, the sharpness of an image reproduced from the obtained image signal changes in accordance with the amount of the stimulating rays, and an image having stable sharpness cannot be obtained.